Add E0695 for unlabeled breaks

src/librustc_passes/diagnostics.rs

@@ -259,6 +259,43 @@ let result = loop { // ok!
     i += 1;
 };
 ```
+"##,
+
+E0695: r##"
+A `break` statement without a label appeared inside a labeled block.
+
+Example of erroneous code:
+
+```compile_fail,E0695
+# #![feature(label_break_value)]
+loop {
+    'a: {
+        break;
+    }
+}
+```
+
+Make sure to always label the `break`:
+
+```
+'l: loop {
+    'a: {
+        break 'l;
+    }
+}
+```
+
+Or if you want to `break` the labeled block:
+
+```
+# #![feature(label_break_value)]
+loop {
+    'a: {
+        break 'a;
+    }
+    break;
+}
+```
 "##
 }
 

src/librustc_passes/loops.rs

@@ -109,6 +109,14 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
                 }
 
                 if self.cx == LabeledBlock {
+                    if label.label.is_none() {
+                        struct_span_err!(self.sess, e.span, E0695,
+                                        "unlabeled `break` inside of a labeled block")
+                            .span_label(e.span,
+                                        "`break` statements that would diverge to or through \
+                                        a labeled block need to bear a label")
+                            .emit();
+                    }
                     return;
                 }
 

src/test/ui/label_break_value_unlabeled_break.rs

@@ -0,0 +1,27 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Simple unlabeled break should yield in an error
+fn unlabeled_break_simple() {
+    'b: {
+        break; //~ ERROR unlabeled `break` inside of a labeled block
+    }
+}
+
+// Unlabeled break that would cross a labeled block should yield in an error
+fn unlabeled_break_crossing() {
+    loop {
+        'b: {
+            break; //~ ERROR unlabeled `break` inside of a labeled block
+        }
+    }
+}
+
+pub fn main() {}

src/test/ui/label_break_value_unlabeled_break.stderr

@@ -0,0 +1,15 @@
+error[E0695]: unlabeled `break` inside of a labeled block
+  --> $DIR/label_break_value_unlabeled_break.rs:14:9
+   |
+LL |         break; //~ ERROR unlabeled `break` inside of a labeled block
+   |         ^^^^^ `break` statements that would diverge to or through a labeled block need to bear a label
+
+error[E0695]: unlabeled `break` inside of a labeled block
+  --> $DIR/label_break_value_unlabeled_break.rs:22:13
+   |
+LL |             break; //~ ERROR unlabeled `break` inside of a labeled block
+   |             ^^^^^ `break` statements that would diverge to or through a labeled block need to bear a label
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0695`.